Silicone hydrogel contact lenses have become popular due to the ability of contact lens wearers to wear such lenses on their eyes for longer times compared to non-silicone hydrogel contact lenses. For example, depending on the particular lens, silicone hydrogel contact lenses can be worn or prescribed for daily wear, weekly wear, biweekly wear, or monthly wear. Benefits to lens wearers associated with silicone hydrogel contact lenses can be attributed, at least in part, to the combination of hydrophilic components and the hydrophobic properties of silicon-containing polymeric materials of the contact lenses.
Non-silicone hydrogel contact lenses, such as 2-hydroxyethylmethacrylate (HEMA) based hydrogel contact lenses, are often produced in non-polar resin contact lens molds, for example, contact lens molds produced from polyolefin-based resins. In other words, lens precursor compositions for non-silicone hydrogel contact lenses are polymerized in non-polar resin contact lens molds to produce HEMA-based polymeric or polymerized lens products. Due to the hydrophilic nature of the polymeric components of HEMA-based contact lenses, the HEMA-based lenses are ophthalmically compatible and have ophthalmically acceptable surface wettabilities, even in spite of being produced using non-polar resin molds.
In contrast, existing silicone hydrogel contact lenses obtained from non-polar resin molds have hydrophobic lens surfaces. In other words, the surfaces of such silicone hydrogel contact lenses have low wettability and therefore are not ophthalmically compatible or ophthalmically acceptable. For example, such silicone hydrogel contact lenses may be associated with increased lipid deposition, protein deposition, lens binding to the ocular surface, and general irritation to a lens wearer.
In an effort to overcome these problems, surface treatment or surface modification of such silicone hydrogel contact lenses or lens products has been employed in an attempt to increase the hydrophilicity and wettability of the lens surfaces. Examples of surface treatment of silicone hydrogel lenses include coating a surface of the lens, adsorbing chemical species onto the surface of the lens, and altering the chemical nature or electrostatic charge of chemical groups on the surface of the lens. Surface treatments have been described which include using a plasma gas to coat the surface of a polymerized lens, or using a plasma gas on a contact lens mold surface to treat the mold prior to forming a polymerized lens. Unfortunately, several drawbacks are associated with this approach. Surface treatment of contact lenses requires more machinery and time to produce contact lenses compared to manufacturing methods that do not use surface treatments or modifications. In addition, surface treated silicone hydrogel contact lenses can exhibit a decreased surface wettability as the lens is being worn and/or handled by the lens wearer. For example, increased handling of a surface treated lens can result in the hydrophilic surface being degraded or worn away.
An alternative approach to increasing the wettability and ophthalmic compatibility of silicone hydrogel lenses is to polymerize a silicone hydrogel contact lens precursor composition in the presence of a second composition that comprises a polymeric wetting agent, such as polyvinylpyrollidone (PVP). These types of lenses are referred to herein as silicone hydrogel contact lenses with polymeric internal wetting agents, and typically comprise an interpenetrating polymer network (IPN) that includes a high molecular weight polymer, such as PVP. As understood by persons of ordinary skill in the art, an IPN refers to a combination of two or more different polymers, in network form, at least one of which is synthesized and/or cross-linked in the presence of the other without any covalent bonds between them. An IPN can be composed of two kinds of chains forming two separate networks, but in juxtaposition or interpenetrating. Examples of IPNs include sequential IPNs, simultaneous IPNs, semi-IPNs and homo-IPNs. Although silicone hydrogel contact lenses that include an IPN of a polymeric wetting agent avoid the problems associated with surface treatment, these lenses may not retain their ophthalmic compatibility, including surface wettability, for prolonged periods of time. For example, the internal wetting agents, since they are not covalently bound to the other polymerized lens forming components, may leach out from the lens while being worn by a lens wearer, and thereby lead over time to a decreased surface wettability and increased discomfort to the lens wearer.
As an alternative to surface treatment or use of a polymeric wetting agent IPN, as described above, it has been found that silicone hydrogel contact lenses with ophthalmically acceptable surface wettabilities can be produced using polar resin molds instead of non-polar resin molds. For example, silicone hydrogel contact lenses formed in ethylene-vinyl alcohol or polyvinyl alcohol based molds have desirable surface wettabilities. One example of a useful polar resin used in the manufacture of contact lens molds for producing non-surface treated silicone hydrogel contact lenses free of an IPN of a polymeric wetting agent is a resin of ethylene-vinyl alcohol copolymers such as the ethylene-vinyl alcohol copolymer resin sold under the trade name SOARLITE™ by Nippon Gohsei, Ltd. In addition to its polarity, SOARLITE™ is described as possessing the following characteristics: extremely high mechanical strength, antistatic properties, low contractility when used in molding processes, excellent oil and solvent resistance, small coefficient of thermal expansion, and good abrasion resistance.
Although SOARLITE™-based molds provide a desirable alternative for producing ophthalmically compatible silicone hydrogel contact lenses without the use of a surface treatment or a polymeric wetting agent IPN, SOARLITE™ molds are less deformable or flexible than non-polar resin molds, such as polypropylene molds, and are relatively more difficult to work with compared to non-polar resin molds.
In view of the above, it can be seen that a need exists for ophthalmically compatible silicone hydrogel contact lenses that can be more easily produced compared to silicone hydrogel contact lenses obtained from SOARLITE™ contact lens molds, and that do not require surface treatment or use of a polymeric wetting agent IPN, including a PVP IPN, to achieve ophthalmic compatibility. Additionally, it would be highly desirable to provide a method for producing an ophthalmically compatible silicone hydrogel contact lens, such as a silicone hydrogel contact lens having an ophthalmically compatible surface wettability, from non-polar resin or polyolefin-based contact lens mold members, which overcomes the disadvantages of existing approaches. That is to say, there is a need for an improved method for preparing an ophthalmically compatible silicone hydrogel contact lens that requires neither surface treatment of the resulting contact lens product nor the use of a polymeric wetting agent IPN as part of a polymerizable silicone hydrogel contact lens precursor composition. The present invention meets these needs.